In 2002, my aircraft carrier battle group was conducting retaliatory strikes against Afghanistan when I was directed to move into the Persian Gulf and begin precursor combat operations against Iraq. While meeting with the fleet commander, I told him we were ready to go. But when asked my opinion of the impeding war, I replied that it was "a tragic misadventure."

Today, if asked about the current combat air sorties over Iraq, my answer would be different.

In 2002, there was no clear or present danger to U.S. security from Iraq. The mission was therefore ambiguous: Initially, it was to seize Iraq's weapons of mass destruction, a dubious claim. When they weren't found, the mission became democracy, and when that failed, it evolved to stability. Without a concrete endgame, there were no benchmarks to measure progress toward a goal. As a result, the mission kept changing and the cost-benefit gap kept expanding.

I felt similarly about our intervention in Libya last year: What was the endgame? How would we measure our mission? The continued chaos in Libya today begs the question of whether it was thought through.

But this time in Iraq, our mission can be limited, with progress measured to determine whether the costs are worth the security benefits. That's because the primary mission is not the removal and reconstruction of the Iraqi government; our real undertaking is to stop the radical terrorist group, ISIS (also known as Islamist State), from having a safe haven in Iraq.

Unlike the questionable intelligence about non-existent weapons of mass destruction in 2003, there is clear evidence that the well-funded ISIS could become an incubator and exporter of terrorism. Already, groups linked to al-Qaida are moving to areas seized by ISIS.

Three objectives are important to safeguard our nation against a safe haven for ISIS in Iraq:

•Stop the ISIS advance, which our military has been doing with surgical intelligence-based airstrikes with limited cost and clear benefit.

•Break logistical support centers within Iraq and the ungoverned regions of Syria that could sustain the extended forces of ISIS — again, something limited military assets can do "from above." Then, over time and with proper arms, the Kurds and Iraqi ground forces can rollback a weakened ISIS force.

•Follow the money and stanch its flow — again, based on intelligence. ISIS is successful because of its deep pockets and murky donors. This last objective takes allies and friends overseas and our domestic agencies acting globally.

Although new dangers originate abroad, the 21st century national security environment requires our Treasury Department and other homeland agencies — not just our military — to work in an international, cooperative effort with other nations' banking institutions and intelligence agencies to stop terrorism's financial support networks.

That is why those who decry our engagement overseas do our security harm. I understand why many Americans are tired of conflict and want to focus solely on affairs at home. However, to replace our international engagement with withdrawal from the world is at the peril of our security.

What we need is to overhaul our overall national security strategy. But in doing so, we must face hard truths.

First, there is no longer a barrier between foreign and domestic policy. They are inextricably entwined in a single national security policy. What happens in a Middle East desert or a Southeast Asia strait cannot but impact Silicon Valley or the Rust Belt — from the flow of money to the flow of oil.

Second, for decades our national security strategy has required that the size of our military be premised on the ability to win two large wars simultaneously. Today's national security environment requires an ability to employ multiple tools against a multitude of threats.

Increasingly, we cannot rely on military tools for two outdated wars, and we must shift toward an interagency capability to implement the widest range of security tools — from Treasury and National Security Agency to the military and Homeland Defense.

Third, today's national security strategy must capture a spectrum of challenges along which we can operate seamlessly from dealing with the emergence of China as a superpower to disrupting terrorist plots. To do this, a new global force posture based on the capability provided by knowledge superiority must replace our force structure based on size.

The threat posed by ISIS exemplifies our need for a posture based on the knowledge-capability from an entire range of national security instruments to both destroy key nodes of ISIS arms and to dry up their money with a relatively small force size.

The defeat of ISIS will depend on our knowledge superiority by command of cyberspace and a tailored global engagement that permits a smaller force to operate seamlessly and with assuredness against ISIS — and along the entire security spectrum.

Joe Sestak is a former Navy admiral and a former congressman from Pennsylvania's 7th District.